THE blast that killed Privates James Kerr and Scott Kennedy, both 20, and Corporal Paul Joszko, 28, would have been heard across half of Basra.

It happened at 1am on Thursday in the deserted streets of al-Antahiya, on the southern outskirts of the city.

The men would have known their mission was dangerous. They had left the relative security of Basra air station - Britain's main base in Iraq, which is spread across a wide expanse of desert bordering the city's airport - to resupply the only British contingent still within the city limits, at Basra Palace. There are only a few possible routes between the two bases, as local insurgents well know, and most journeys are undertaken by helicopter.

To minimise the risk of travelling by road, the convoy had gone to Basra Palace in darkness, when ordinary residents of the city remain indoors behind high walls. The only people who venture out are British troops - and those they are fighting. The soldiers had delivered their supplies and were on the way back when they left their Warrior armoured vehicles to check their surroundings. At this point, it appears, a hidden watcher triggered the bomb that killed the three men and seriously wounded a fourth.

The loss of three soldiers in one attack, the second worst in Iraq so far this year, brought the British toll since the 2003 invasion to 156. Not only did it come on Gordon Brown's first full day as prime minister, one of the victims, Pvt Kerr, was from Cowdenbeath in Brown's constituency.

In significant contrast to his predecessor Tony Blair, who never had contact with relatives of British soldiers lost in Iraq or Afghanistan, Brown telephoned Kerr's mother to express his condolences. Some are wondering whether that signals a change of approach on Iraq; if he were to attend the repatriation ceremony or the soldier's funeral, it would send the most eloquent of messages.

Brown's view on Iraq The new PM was never expected to continue Blair's symbiotic relationship with George Bush, his partner in war, but Brown's government appointments removed any doubt.

David Miliband, his Foreign Secretary, privately doubted the wisdom of invading Iraq. Mark Malloch Brown, made a Foreign Office minister, made no secret of his view in his previous job - deputy UN secretary-general - that the war was almost certainly illegal.

The fact that Des Browne, the Defence Secretary, was the only minister to remain in the same post after Brown's extensive reshuffle may indicate that the new incumbent in No 10 saw the need for some continuity in Britain's wars. But the situation on the ground in southern Iraq carries its own momentum, one that appears to point to an early exit for British troops.

In one of the most detailed independent reports on Basra since the invasion, the authoritative International Crisis Group (ICG) last week painted a devastating portrait of life in the city. It said Operation Sinbad, Britain's attempt between September 2006 and March this year to root out militias, restore security and kick-start economic reconstruction, appeared at first to be a qualified success. Criminality, political assassinations and sectarian killings that were rampant in 2006 receded somewhat, and relative calm prevailed.

"Yet this reality was both superficial and fleeting, " says the ICG report.

"By March-April 2007, renewed political tensions once more threatened to destabilise the city, and relentless attacks against British forces in effect had driven them off the streets into increasingly secluded compounds.

Basra's residents and militiamen view this not as an orderly withdrawal, but rather as an ignominious defeat.

Today the city is controlled by militias, seemingly more powerful and unconstrained than before."

'It already may well be too late' The ICG blames Britain for "the most glaring failing of all": the inability to establish a strong provincial administration capable of enforcing its will.

Instead of the political parties responsible for the violence having been confronted, they were treated as partners .

The report adds: "In any event, time is running out. Four years after the fall of Saddam Hussein's regime, they are facing increasingly frequent and bloody attacks, and it is hard to imagine them remaining for long."

Even if the coalition wanted to reengage, says the ICG "it already may well be too late".

That was clear the minute Blair, going against US wishes, announced earlier this year that British forces in Iraq would be reduced. The bitter irony of last week's attack is that the resupply convoy on which the three soldiers died may well have been the last to Basra Palace, which is due to be handed over to Iraqi forces any day now. That will leave the airport as the only British base in Iraq.

Full withdrawal impossible Military sources believe another 500 troops may come home soon, but that would be the last partial withdrawal: 5,000 is considered the minimum number needed to ensure that the remaining force can protect itself. But what would be the point, many ask, of keeping them in one location, unable to achieve much beyond acting as a magnet for insurgent attacks?

Critics argue that a precipitate British withdrawal would make the violence in Basra even worse. The diminished British presence has created worsening friction with US commanders, who are concerned about the security of the supply route from Kuwait.

Even without such pressures, it was impossible to contemplate the end of the British mission in Iraq while Tony Blair remained prime minister. Nor is his successor likely to make any announcement of withdrawal in the near future. The most likely outcome is a quiet handover of security in Basra to the local administration and the Iraqi army.

Most British troops would leave immediately afterwards, leaving behind perhaps 1,500 to carry out training duties in the desert. This weekend, for the first time since 2003, it seems possible to pose the uncomfortable question: who will be the last British soldier to die in Iraq?